1. Technical Field
This invention relates to audio systems and, more particularly, to systems for selectively connecting speech circuits to an audio line in response to voice signals.
2. Description of the Prior Art
Many companies now consider teleconferencing as a cost effective way of communicating among personnel at dispersed locations and thereby reduce the need for business travel. In an audio teleconferencing arrangement, a number of conferees at a location are placed in communication with a number of conferees at one or more remote locations via a telephone connection. The quality of the transmission between the separated groups of conferees is generally dependent upon the position of each conferee with respect to a microphone and loudspeaking device at each location. With a single microphone and loudspeaking device in the conference location room, the transmission is subject to degradation because some of the conferees are generally at a greater than optimum distance from the microphone and loudspeaking device.
It is well known to use a plurality of microphones appropriately spaced at each conferee location to improve the quality of the conference system. The microphone outputs are summed and the summed output is applied to the communication links between locations. In such an arrangement, each conferee can be within an acceptable distance from one of the microphones, whereby speech pickup is of relatively good quality. With all microphones turned on at one time, however, several undesirable effects occur. The total noise pickup is much greater than for a single microphone. The artificial reverberation effects occasioned by the delayed signal pickup from the more remote microphones severely lower the quality of the conference transmission. Further, electroacoustic instability can easily result from the plurality of the always turned on microphones. It is therefore desirable and known in the art to provide a switching arrangement which permits only that microphone closest to the talking conferee to be active so that reverberation and noise pickup are minimized.
Such an arrangement is commonly known as a "voting circuit." In the "voting circuit" arrangement, the loudest talker can capture control and lock out the other conferees at his or her location. This automatic switching between microphones responsive to the highest speech level microphones, however, may also result in transmission interruptions which adversely affect intelligibility and can result in unwanted interference occasioned by transient room noise. For example, a loud noise at one of the conference locations may completely turn off the controlling microphone. Further, since only one microphone is operative at a time, transfer of control from one microphone to another such as occasioned by the talking conferee moving from one position to another in a room location can result in speech transmission of varying quality, interruptions in transmission, and reverberation effects which vary with the talking conferee's position.
Various teleconferencing arrangements have been proposed and used heretofore for selecting a single microphone of a plurality of conferee microphones and for transmitting the signal from only the selected microphone. An example of such an arrangement is seen in U.S. Pat. No. 3,730,995, issued to M. V. Matthews on May 1, 1973. In this arrangement, each of a plurality of microphones is associated with a speech detector and a relay. In response to voice signals from one of the microphones, an associated speech detector activates its relay which connects the microphone to an audio line and generates a signal inhibiting the other relays. Another example is seen in U.S. Pat. No. 3,755,625, issued to D. J. Maston on Aug. 28, 1973. This patent discloses a multimicrophone-speakerphone arrangement using a comparator in combination with logic circuitry for selecting a microphone with the greatest output and connecting it to the speakerphone input while simultaneously disconnecting the other microphones.
Still another example is seen in U.S. Pat. No. 4,449,238, issued to B. H. Lee, et al. on May 15, 1984. This patent discloses a computer based sound system wherein a microphone with the greatest output level is "selected" while all others are either attenuated or off. Yet still another example is seen in U.S. Pat. No. 4,658,425 issued to S. D. Julstrom on Apr. 14, 1987. This patent discloses a microphone actuation control system in which three first-order-gradient (FOG) microphones, each having a heart-shaped (cardioid) polar response pattern, share a common housing with a loudspeaker. Each of the microphones faces outward so that the direction of maximum sensitivity emanates radially from the center of the housing. The overall pattern provided by the three microphones allows full room (360.degree.) coverage, although normally only one microphone may be on. In the absence of local speech, each of these microphones is gated off. Unfortunately, some level of syllabic clipping occurs when a microphone turns on from a full off condition.
While these arrangements have been satisfactory in minimizing the degradation of the speech signals due to reverberation and noise pickup, it is nevertheless desirable to make the microphone selection technique appear to occur in as normal a manner as possible. That is, not only should the microphone voting circuit recognize and respond to the loudest conferee in the room as do the other conferees, but it should also allow other conferees in the room who speak simultaneously with the loudest conferee to be heard quickly and equally well by conferees at a remote location. And it is also desirable to avoid the syllabic clipping that occurs when a microphone turns on from the full off condition.